Fluence-response relationships for the enhancement of ultraweak luminescence by UV radiation were determined in leaves of Brassica napus L. (cv. Ceres) when either the adaxial or abaxial leaf surface was exposed to monochromatic radiation (270–340 nm). Action spectra for both leaf surfaces were constructed from the fluence-response curves at the fluence required to obtain a doubling of the ultraweak luminescence level over the unirradiated controls. The action spectrum for the abaxial leaf surface showed a steeper slope than that for the adaxial leaf surface at wavelengths below 280 nm. Both action spectra were flatter than the generalized plant action spectrum of Caldwell (M.M. Caldwell, in A.C. Giese (ed.), Photophysiology, Vol. 6, Academic Press, New York, 1971, pp. 131–177) at wavelengths above 280 nm. Under a depleted ozone layer, the action spectra yielded a radiation amplification factor approximately 50% of that obtained with Caldwell's generalized plant action spectrum. The shape of the action spectrum for the induction of ultraweak luminescence was approximately the same for both surfaces above 290 nm.